Periodic Boundary class for 2 target domains

variational formulation
1

Hey there,

my domain is [0, length] x [0, length] x [0,length] cube created with mshr. I want that the opposite xz and yz planes of the domain are periodic and did the following:

class PeriodicBoundary(SubDomain):
def inside(self, y, on_boundary):
    #left and front boundary are target domain
    return bool((near(y[0],0) and on_boundary) \
            or (near(y[1],0) and on_boundary)) \
            and (not(near(y[0],length) or near(y[1],length)))

def map(self,x,y):
    #map right and back boundary to target domain
    if near(x[0], length):
        y[0] = x[0] - length
        y[1] = x[1]
        y[2] = x[2]
    elif near(x[1], length):
        y[0] = x[0]
        y[1] = x[1] - length
        y[2] = x[2]
    else:
        y[0] = -1000
        y[1] = -1000
        y[2] = -1000​

I included the else condition because I get otherwise the following error:

*** -------------------------------------------------------------------------
*** DOLFIN encountered an error. If you are not able to resolve this issue
*** using the information listed below, you can ask for help at
***
***     fenics-support@googlegroups.com
***
*** Remember to include the error message listed below and, if possible,
*** include a *minimal* running example to reproduce the error.
***
*** -------------------------------------------------------------------------
*** Error:   Unable to periodic boundary mapping.
*** Reason:  Need to set coordinate 0 in sub_domain.map.
*** Where:   This error was encountered inside PeriodicBoundaryComputation.cpp.
*** Process: 0
*** 
*** DOLFIN version: 2018.1.0
*** Git changeset:  948dc42cc4e06ed9227d0201ad50f94ac94cbf9f
*** -------------------------------------------------------------------------

I found this, which was my guidance: https://fenicsproject.org/qa/262/possible-specify-more-than-one-periodic-boundary-condition/

Why do I have to include the else condition? Even with the else condition the code doesn’t work as I want to. I tried to set one of each periodic pair to 0, but the dofs are not mapped to each other and therefore only one wall is 0. What am I missing?

I copied this from the allanswered.

Thanks in advance
Kind regards
Max

2

You should first check the code with a mesh which you know is periodic (e.g. ´UnitCubeMesh´). I suspect yours is not.

3

Thanks MiroK for your comment. I’ve tried it with the UnitCubeMesh and indeed the periodic b.c. works but only with the else statement. Otherwise I get the very same error. Can you tell me why the else is needed?

Thanks and kind regards
Max

4

Else clause is there to have the point always end up mapped somewhere, y \neq nan here.

5

Hi, Do you know how we can have periodic boundary conditions for aperiodic mesh?